Method for preventing explosion of exhaust gas in decompression processing apparatus

ABSTRACT

Disclosed is a plasma processing apparatus in which a main control unit is capable of managing the processing situation of an exhaust gas in an exhaust gas processing unit through a dilution controller. The exhaust gas processing unit includes a detoxifying device connected to the outlet of a vacuum pump through an exhaust pipe, a dilution gas source connected to the exhaust pipe near the outlet of the vacuum pump through a dilution gas supply pipe, an MFC and an opening/closing valve installed at the middle of the dilution gas supply pipe, a gas sensor attached to the exhaust pipe on the downstream side of an end (node N) of the dilution gas supply pipe, and a dilution controller configured to control the MFC.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority from Japanese Patent Application No. 2014-101048, filed on May 15, 2014, with the Japan Patent Office, the disclosure of which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

The present disclosure relates to a method for preventing explosion of an exhaust gas at the outlet side of a vacuum pump that evacuates an inside of a chamber of a decompression processing apparatus.

BACKGROUND

In a process of manufacturing a semiconductor device, various gases are used in a process performed on a semiconductor substrate under a reduced pressure. Conventionally, in a decompression processing apparatus using a vacuum chamber, more particularly, a plasma processing apparatus, a combustible gas and a combustion supporting gas may be simultaneously or alternately supplied into the same chamber. In such a case, preventing explosion of an exhaust gas in advance at the outlet side of a vacuum pump that evacuates the inside of a chamber has become one of the problems to be solved.

For example, in a plasma CVD apparatus, after a film is formed on a semiconductor substrate through a plasma CVD method using a combustible raw material gas in a chamber, plasma cleaning for removing deposits adhered to an inner wall of the chamber with plasma of a combustion supporting gas may be performed every time or periodically. In this case, when some of the combustible gas used in the film forming processing remains unreacted in the chamber and is mixed with the combustion supporting gas and discharged from the chamber during the immediately following plasma cleaning, a high-pressure or high-concentration exhaust gas may cause explosion (rapid pressure variation through combustion) at the outlet side of a vacuum pump due to, for example, generation of static electricity. In general, a detoxifying device is connected to the outlet side of the vacuum pump used in the plasma processing apparatus through an exhaust pipe. Since a vacuum pipe which is poor in strength and pressure resistance is used as this kind of exhaust pipe, the exhaust pipe may be easily damaged by the explosion of the exhaust gas so that an exhaust gas harmful to, for example, the human body or the environment may leak out to the outside.

In addition, a plasma etching apparatus also has a problem in that, when a dry etching is performed using a mixed gas composed of a combustible gas and a combustion supporting gas, a high-pressure or high-concentration exhaust gas containing the unreacted combustible gas and the combustion supporting gas may cause explosion at the outlet side of a vacuum pump, thus leaking out to the outside of an exhaust pipe.

Conventionally, a technology has been known in which, in order to prevent, in advance, explosion of an exhaust gas at the outlet side of a vacuum pump that evacuates the inside of a chamber of a decompression processing apparatus, an inert gas such as, for example, N₂ gas is supplied to an exhaust pipe that connects the outlet side of the vacuum pump to an exhaust gas processing device such as, for example, a detoxifying device so as to dilute the exhaust gas (see, e.g., Japanese Patent Laid-Open Publication No. H9-000909 and Japanese Patent Laid-Open Publication No. 2012-038962).

SUMMARY

The present disclosure provides a method for preventing explosion of an exhaust gas in an exhaust path at the outlet side of a vacuum pump included in a decompression processing apparatus, the decompression processing apparatus including a chamber to which a combustible gas including CH₄ (methane) gas and a combustion supporting gas including a halogen element are simultaneously or alternately introduced to perform a process on a processing target substrate at a reduced pressure, the vacuum pump being configured to evacuate an inside of the chamber, the method including: supplying an inert dilution gas to the exhaust path or the vacuum pump and controlling a supply flow rate of the inert gas such that the CH₄ gas in the exhaust path has a concentration in a range of 0.05% to 0.10%.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, exemplary embodiments, and features described above, further aspects, exemplary embodiments, and features will become apparent by reference to the drawings and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically illustrating a configuration of an exemplary plasma processing apparatus to which a method for preventing explosion of an exhaust gas of the present disclosure may be applied.

FIG. 2 is a table illustrating results of an evaluation test according to an exemplary embodiment.

FIG. 3 is a view illustrating, for example, a magnitude relationship of bond dissociation energy between a raw material side and a product side in a chemical reaction formula for eight specific kinds of combustible gases.

FIG. 4 is a view illustrating, for example, a molecular structure and a chemical reaction formula of each of specific combustible gases (belonging to a second group) classified from the results of the evaluation test.

FIG. 5 is a graph illustrating a characteristic curve (regression curve) of a lower explosion concentration limit acquired through a regression analysis method based on plots obtained from the evaluation test for the combustible gases belonging to the second group.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. The illustrative embodiments described in the detailed description, drawing, and claims are not meant to be limiting. Other exemplary embodiments may be utilized, and other changes may be made without departing from the spirit or scope of the subject matter presented here.

The conventional method for preventing explosion of an exhaust gas as described above is a technology that prevents explosion by a combustible gas alone or explosion by a combustion supporting gas alone but not a technology that prevents explosion of an exhaust gas in which a combustible gas and a combustion supporting gas are mixed. In a case where a combustible gas and a combustion supporting gas are mixed in an exhaust gas, it is important to have a proper knowledge about presence or absence of an explosion probability or about an explosion condition according to the kinds and combinations of gases, and to have precise numerical information related to a lower explosion limit of the gas concentration when there is an explosion probability. Otherwise, gas leakage may occur due to unexpected explosion of the exhaust gas, or unnecessary consumption (waste) of a dilution gas may be caused.

In particular, in recent etching processes, processes that need to simultaneously supply a combustible gas and a combustion supporting gas to a processing container are increasing (see, e.g., Japanese Patent Laid-Open Publication No. 2012-204408). For this reason, a plasma etching apparatus that performs an etching process using such a mixed gas requires a function or equipment for preventing explosion of an exhaust gas which is efficient and highly stable compared to the related art.

The inventors of the present disclosure performed an evaluation test for several combinations of a combustible gas and a combustion supporting gas that are generally used in a semiconductor device manufacturing process and are potentially mixed in an exhaust system or a vacuum chamber of a decompression processing apparatus.

The present disclosure is based on the above-described evaluation test and provides a method for preventing explosion of an exhaust gas in a decompression processing apparatus which optimizes the use efficiency of an inert dilution gas used for preventing explosion of an exhaust gas, in advance, at the outlet side of a vacuum pump of an exhaust system with respect to a combination of a specific combustible gas (CH₄, CH₃F, or CH₂F₂) and a halogen-based combustion supporting gas which may explode when mixed.

According to a first aspect of the present disclosure, a method for preventing explosion of an exhaust gas in an exhaust path at the outlet side of a vacuum pump included in a decompression processing apparatus, the decompression processing apparatus including a chamber to which a combustible gas including CH₄ (methane) gas and a combustion supporting gas including a halogen element are simultaneously or alternately introduced to perform a process on a processing target substrate at a reduced pressure, the vacuum pump being configured to evacuate an inside of the chamber, includes: supplying an inert dilution gas to the exhaust path and controlling a supply flow rate of the inert gas such that the CH₄ gas in the exhaust path has a concentration in a range of 0.05% to 0.10%.

According to a second aspect of the present disclosure, a method for preventing explosion of an exhaust gas in an exhaust path at the outlet side of a vacuum pump included in a decompression processing apparatus, the decompression processing apparatus including a chamber to which a combustible gas including CH₃F (fluoromethane) gas and a combustion supporting gas including a halogen element are simultaneously or alternately introduced to perform a process on a processing target substrate at a reduced pressure, the vacuum pump being configured to evacuate an inside of the chamber, includes: supplying an inert dilution gas to the exhaust path and controlling a supply flow rate of the inert gas such that the CH₃F gas in the exhaust path has a concentration in a range of 3.0% to 5.0%.

According to a third aspect of the present disclosure, a method for preventing explosion of an exhaust gas in an exhaust path at the outlet side of a vacuum pump included in a decompression processing apparatus, the decompression processing apparatus including a chamber to which a combustible gas including CH₂F₂ (difluoromethane) gas and a combustion supporting gas including a halogen element are simultaneously or alternately introduced to perform a process on a processing target substrate at a reduced pressure, the vacuum pump being configured to evacuate an inside of the chamber, includes: supplying an inert dilution gas to the exhaust path and controlling a supply flow rate of the inert gas such that the CH₂F₂ gas in the exhaust path has a concentration in a range of 8.3% to 13.3%.

In a method for preventing explosion of an exhaust gas of the present disclosure, in a case where any one of CH₄ gas, CH₃F gas or CH₂F₂ gas and a halogen-based combustion supporting gas are included in an exhaust gas at the outlet side of a vacuum pump, the supply flow rate of an inert gas to be supplied to an exhaust pipe is controlled such that the concentration of the combustion supporting gas is kept at or near a value lower than a lower explosion limit.

According to the method for preventing explosion of an exhaust gas of the present disclosure, through a configuration as described above, it is possible to optimize the use efficiency of an inert dilution gas, used to prevent explosion of an exhaust gas, in advance, at the outlet side of a vacuum pump of an exhaust system, with respect to a combination of a specific combustible gas (CH₄, CH₃F, and CH₂F₂) and a halogen-based combustion supporting gas which have an explosion probability when mixed.

Hereinafter, a method for preventing explosion of an exhaust gas according to exemplary embodiments of the present disclosure will be described with reference to the accompanying drawings.

[Exemplary Decompression Processing Apparatus of Exemplary Embodiment]

FIG. 1 illustrates an exemplary decompression processing apparatus to which a method for preventing explosion of an exhaust gas according to the present disclosure may be applied. The decompression processing apparatus 1 is configured as a capacitively coupled plasma processing apparatus and includes a vacuum chamber 10 made of aluminum and may withstand a reduced pressure. For example, the inner wall of the vacuum chamber 10 is anodized. The chamber 10 is grounded and a sidewall of the chamber is formed with a carry-in/carry-out opening (not illustrated) for introduction/discharge of a processing target substrate, for example, a semiconductor wafer W.

A conductive susceptor 14 made of, for example, aluminum is installed on the bottom of the chamber 10 through a cylindrical insulation member 12. The susceptor 14 configures a lower electrode and the semiconductor wafer W—a processing target object—is mounted on the susceptor. In addition, the susceptor 14 serves as a high frequency electrode and a high frequency power source 16 that outputs a high frequency power for plasma generation is electrically connected to the susceptor 14 through a matcher 18. In addition, the susceptor 14 includes, for example, a holding mechanism (not illustrated) for holding the semiconductor wafer W or a temperature control mechanism (not illustrated) for controlling the temperature of the wafer W.

An upper electrode 20 is installed above the susceptor 14 and directly attached to the ceiling of the chamber 10 to be opposite to and in parallel with the susceptor 14. The upper electrode 20 is configured as a showerhead and has a plurality of or innumerable through-holes 20 a for ejecting a processing gas toward the substrate W on the susceptor 14 and a gas buffer room 20 b for obtaining a uniform gas ejection pressure. A gas supply pipe 26 from a processing gas supply unit 24 is connected to a gas inlet port 22 of the showerhead 20.

The processing gas supply unit 24 includes n gas sources G₁ to G_(n), which individually deliver n kinds of gases (n being an integer number of 2 or more) that are usable in the plasma processing apparatus. Output ports of these gas sources G₁ to G_(n) are respectively connected to the common gas supply pipe 26 through individual electromagnetic opening/closing valves and mass flow controllers (MFCs). A main opening/closing valve 28 is installed to the gas supply pipe 26.

The chamber 10 has one exhaust hole or a plurality of exhaust holes 30 formed in the bottom thereof. The exhaust hole(s) 30 is connected to a vacuum pump 36 through an exhaust pipe 32 and a pressure regulator, for example, an APC valve 34. The vacuum pump 36 may be a single pump, or may be a combination of a plurality of pumps such as, for example, an auxiliary (roughing) pump or a main (high-vacuum) pump. The pressure regulator 34 regulates the interior pressure of the chamber 10 by regulating the flow rate of an exhaust gas. An exhaust gas may be discharged out of the chamber 10 through the exhaust hole(s) 30 from the periphery of the susceptor 14 by the pressure regulator 34 and the vacuum pump 36 and a processing space in the chamber 10 may be decompressed to a desired vacuum.

An exhaust gas processing unit 40 is provided at the outlet side of the vacuum pump 36. A configuration and operation of the exhaust gas processing unit 40 will be described later in detail.

In the plasma processing apparatus as described above, when a plasma processing is performed on the semiconductor wafer W on the susceptor 14, one kind of or a plurality of kinds of preselected gases from the processing gas supply unit 24 is introduced into the chamber 10 through the showerhead 20 at a predetermined flow rate and the interior pressure of the chamber 10 is controlled to a set value by the pressure regulator 34 and the vacuum pump 36. In addition, a high frequency power of a predetermined frequency which is suitable for plasma generation is applied from the high frequency power source 16 to the susceptor (lower electrode) 14 through the matcher 18, with a predetermined power. A processing gas ejected from the through-holes 20 a of the showerhead (upper electrode) 20 to the processing space in the chamber 10 generates plasma via high frequency discharge (glow discharge) between the lower and upper electrodes 14 and 20, and a plasma processing such as, for example, etching or deposition, is performed on a main surface (upper surface) of the semiconductor wafer W by, for example, radicals or ions included in the plasma.

A main control unit 42 includes a microcomputer and various interfaces and controls operations of respective components in the plasma etching apparatus such as, for example, an exhaust unit (the pressure regulator 34 and the vacuum pump 36), a high frequency power supply unit (the high frequency power source 16 and the matcher 18), the processing gas supply unit 24, the main opening/closing valve 28, and the exhaust gas processing unit 40, as well as the operation (sequence) of the entire apparatus according to software (programs) and recipe information stored in an external memory 44 or an internal memory.

In the plasma processing apparatus as described above, for example, the kinds, combinations, flow rates, and flow rate ratios (mixing ratios) of gases supplied from the processing gas supply unit 24 to the chamber 10 are preset according to the specifications of a process to be performed and are included, as a part of a process condition, in a recipe. Then, when the process is executed, the gas source, the opening/closing valve, and the MFC in the processing gas supply unit 24, which correspond to one gas or each of a plurality of gases used in the process, are adapted to selectively function. The main control unit 42 checks and manages, for example, the kinds, combinations, flow rates, and flow rate ratios (mixing ratios) of gases supplied from the processing gas supply unit 24 into the chamber 10 during execution of a process based on recipe information.

Meanwhile, the main control unit 42 may also manage the processing situation of an exhaust gas in the exhaust gas processing unit 40 through a dilution controller 45. Here, the exhaust gas processing unit 40 includes a detoxifying device 48 that is connected to an outlet of the vacuum pump 36 through an exhaust pipe 46 formed of, for example, a vacuum pipe, a dilution gas source 52 that may be connected to the exhaust pipe 46 near the outlet of the vacuum pump 36 through a dilution gas supply pipe 50, a thermal type or float type flow rate control mechanism (MFC or flow meter) 54 and an opening/closing valve 55 that are installed at the middle of the dilution gas supply pipe 50, a gas sensor 56 that is attached to the exhaust pipe 46 on the downstream side of an end (node N) of the dilution gas supply pipe 50, and the dilution controller 45 that controls the flow rate control mechanism 54. A dilution gas delivered from the dilution gas source 52 is an inert gas, i.e. N₂ gas or rare gas (e.g., helium or argon).

The detoxifying device 48 is, for example, a dry detoxifying device configured to react a toxic ingredient contained in a collected exhaust gas with a chemical treatment agent so that the toxic ingredient is changed into a safe compound and fixed to the treatment agent. The detoxifying device 48 sets a treatment limit such as, for example, 300 liters per minute, with respect to a possible ventilation amount and also sets an upper limit with respect to the concentration of a gas as a detoxifying target object. Therefore, the flow rate and concentration of an exhaust gas collected by the detoxifying device 48 are managed by appropriately operating, for example, the pressure regulator 34 and the flow rate control mechanism 54 under control of the main control unit 42 or the dilution controller 45.

The exhaust gas processing unit 40 according to the exemplary embodiment is adapted to selectively operate a dilution unit 57 (the dilution gas source 52, the flow rate control mechanism 54, the opening/closing valve 55, and the dilution controller 45) according to the kinds and combinations of a combustible gas and/or a combustion supporting gas included in a high-pressure or high-density exhaust gas discharged from the outlet of the vacuum pump 36. Once the dilution unit 57 has been operated, the exhaust gas processing unit 40 controls the supply flow rate of a dilution gas such that the concentration of a predetermined combustible gas in the exhaust pipe 46 does not exceed a predetermined lower explosion limit and is kept at or near the limit. In this way, consumption of an inert dilution gas used to prevent explosion of an exhaust gas, in advance, in the exhaust pipe 46 may be restricted to a necessary minimal level.

Accordingly, the gas sensor 56 of the exhaust gas processing unit 40 may perform identification of the kind of a gas and measurement of the concentration of a gas at the same time or in real time and, for example, an infrared gas sensor is appropriately used. The infrared gas sensor is configured to determine the kind of a gas from an absorbed wavelength and to measure the concentration of the gas by an absorption intensity because a gas molecule consisting of two or more different atoms absorbs light of a predetermined wavelength within an infrared region (1 μm to 20 μm) that is determined by the molecular structure of the gas molecule. An output signal of the gas sensor 56 is provided to the dilution controller 45. The dilution controller 45 is adapted to control the opening/closing valve 55 according to the kind of a gas included in an exhaust gas based on a control signal from the main control unit 42 and an output signal of the gas sensor 56 and to control the supply flow rate of an inert dilution gas through the flow rate control mechanism 54.

[Evaluation Test of Exemplary Embodiment]

The inventors of the present disclosure have performed an evaluation test on several combinations of a combustible gas and a combustion supporting gas that are generally used in a semiconductor device manufacturing process and can be mixed in an exhaust system or a vacuum chamber of a decompression processing apparatus. Knowledge and numerical information obtained from the evaluation test are reflected, as basic information, in, for example, software used in the main control unit 42 or a recipe stored in the memory 44 in the decompression processing apparatus 1 according to the above-described exemplary embodiment.

In the evaluation test, each of seven selected kinds of combustible gases, i.e. CH₄ (methane) gas, CO (carbon monoxide) gas, CH₃F (fluoromethane) gas, CH₂F₂ (difluoromethane) gas, C₄F₆ (hexafluoro 1,3 butadiene) gas, COS (carbon oxysulfide) gas and C₂H₄ (ethylene) gas, and Cl₂ (chlorine) gas as a representative halogen-based combustion supporting gas were mixed in an experimental chamber. Then, whether a combustion reaction or ignition occurs was tested by varying the concentration of each combustible gas. Then, for combinations having an ignition probability, the lowest explosion concentration (lower explosion limit) of the combustible gas was calculated. An ignition plug of an automobile engine was used as a means for generating flame. A pressure gauge for measurement of an explosion pressure (ZT-15 manufactured by NAGANO GAUGE CORPORATION) was used as a means for detecting ignition. In addition, in the exhaust system of the decompression processing apparatus, electrostatic flame may be generated during rotation of a rotor from, for example, deposits attached to the rotor of the vacuum pump, causing explosion of an exhaust gas at the outlet side of the vacuum pump, which causes gas leakage from the exhaust pipe. FIG. 2 illustrates results of the evaluation test in a table.

In addition, in the present specification, “combustible” means that there is a possibility of combustion. Here, although “combustion” generally refers to a rapid chemical reaction accompanied by heat emission or light emission associated with oxygen, combustion in a broad sense as a chemical term refers to a raid chemical reaction accompanied by heat emission or light emission regardless of whether or not oxygen is present. In addition, “combustion supporting” means the nature to assist combustion of a combustible material. The present disclosure deals with whether a predetermined combustible gas is combusted via mixing with a combustion supporting gas including a halogen gas, i.e. whether there is a rapid reaction between a combustible gas and a combustion supporting gas. Accordingly, in the present specification, the word “combustion” in a broad sense as a chemical term is used.

“Ignition” means that a flame discharge occurs in a mixed gas including a combustible gas and a combustion supporting gas so that the combustible gas and the combustion supporting gas cause a rapid chemical reaction accompanied by heat emission or light emission, i.e. cause combustion.

“Explosion” means that, as a result of a rapid chemical reaction (combustion) accompanied by heat emission or light emission of a combustible gas and a combustion supporting gas, a pressure near a place where the reaction occurs is rapidly raised.

“Lowest explosion concentration” or “lower explosion limit” refers to a lower limit of the concentration of a combustible gas when ignition or explosion occurs under the presence of a combustion supporting gas. In a different view, “lowest explosion concentration” or “lower explosion limit” refers to a critical value in a sense that, when the concentration of a combustible gas is lower than the critical value thereof, the combustible gas does not cause ignition or explosion even if it is mixed with a combustion supporting gas.

As illustrated in FIG. 2, in the evaluation test, a combination of CH₄ gas and Cl₂ gas was ignited (exploded by flame discharge) when the concentration of the CH₄ gas was 0.3%, 0.5%, 1%, 5%, and 5.3%, although merely mixing these gases does not cause a reaction. However, when the concentration of the CH₄ gas was 0.1%, ignition did not occur. Thus, it has been found that the lowest explosion concentration is in a range of greater than 0.1% and 0.3% or less.

In a combination of CO gas and Cl₂ gas, there were no reaction caused merely by mixing and ignition did not occur even by provision of flame discharge in each case where the concentration of CO gas was 12.5%, 30%, 50%, 70% and 90%.

A combination of CH₃F gas and Cl₂ gas did not cause a reaction when these gases were simply mixed with each other. In addition, even ignition did not occur when the concentration of the CH₃F gas was 5%. However, ignition occurred when the concentration of the CH₃F gas was 6.8% or more. Accordingly, it has been found that the lowest explosion concentration of this combination is in a range of greater than 5% and 6.8% or less.

A combination of CH₂F₂ gas and Cl₂ gas did not cause a reaction when these gases were simply mixed with each other. In addition, even ignition did not occur when the concentration of the CH₂F₂ gas was 13.3%. However, ignition occurred when the concentration of the CH₂F₂ gas was in a range of 15% to 20%. Accordingly, it has been found that the lowest explosion concentration of this combination is in a range of greater than 13.3% and 15% or less.

In a combination of C₄F₆ gas and Cl₂ gas, there were no reaction caused merely by mixing and ignition did not occur even by provision of flame discharge in each case where the concentration of the C₄F₆ gas was 7.4%, 20%, 40%, 60% and 85%.

In a combination of COS gas and Cl₂ gas, there were no reaction caused merely by mixing and ignition did not occur even by provision of flame discharge in each case where the concentration of the COS gas was 12%, 30%, 50%, 70% and 90%.

In a combination of C₂H₄ gas and Cl₂ gas, it has been found that, when the concentration of the C₂H₄ gas is in a range of 7.4% to 40%, mixing of these gases sufficiently causes a combustion or explosion reaction (even if flame is not given).

As described above, it has been found from the evaluation test that, among the seven selected kinds of the combustible gases, C₂H₄ gas reacts (combusts) by being merely mixed with Cl₂ gas, CH₄ gas, CH₃F gas and CH₂F₂ gas do not react (combust) when merely mixed with Cl₂ gas, but is ignited by flame when the concentration of each gas is the lower explosion limit or more, and CO gas, COS gas and C₄F₆ gas do not react (combust) when merely mixed with Cl₂ gas and is not ignited even when flame is given.

[Classification of Reaction Form of Combustible Gas and Combustion Supporting Gas of Exemplary Embodiment]

FIG. 3 illustrates reactivity with Cl₂ gas and a magnitude relationship of bond dissociation energy between a raw material side and a product side in a chemical reaction formula, in eight kinds of combustible gases including the above-described seven kinds of combustible gases (CH₄, CO, CH₃F, CH₂F₂, C₄F₆, COS and C₂H₄) and additionally NH₃ (ammonia) gas.

As described above, the seven kinds of combustible gases are classified into three types or groups including “combusts by being merely mixed” (C₂H₄), “is ignited by flame” (CH₄, CH₃F, and CH₂F₂), and “does not combust” (COS, CO and C₄F₆) with regard to reactivity with Cl₂ gas. In addition, it is already known that NH₃ gas causes a combustion reaction by being merely mixed with Cl₂ gas, and NH₃ gas belongs to the same first group as C₂H₄ gas.

As a result of analyzing and examining the results of the evaluation test in view of a theory, the inventors of the present disclosure have found that discrimination of these three groups can be explained in terms of bond dissociation energy as illustrated in FIG. 3. That is, in a chemical reaction formula representing a combustion reaction, comparing bond dissociation energy of interatomic bonding of a raw material side with bond dissociation energy of interatomic bonding of a product side with reference to numerical values disclosed in a chemical handbook, it has been found that bond dissociation energy of interatomic bonding of a product side is obviously greater than bond dissociation energy of interatomic bonding of a raw material side in a first group to which NH₃ gas and C₂H₄ gas belong, bond dissociation energy of interatomic bonding of a product side is approximately equal to bond dissociation energy of interatomic bonding of a raw material side in a second group to which CH₄ gas, CH₃F gas, and CH₂F₂ gas belong, and bond dissociation energy of interatomic bonding of a product side is obviously smaller than bond dissociation energy of interatomic bonding of a raw material side (by at least several % or more) in a third group to which COS gas, CO gas, and C₄F₆ gas belong.

Here, bond dissociation energy means energy required to dissociate a bond of interatomic bonding. In terms of bond dissociation energy, a chemical reaction including combustion may be referred to as a reaction in which interatomic bonding of a gas before reaction is first dissociated and energy is generated when the gas rebonds with atoms of a combustion supporting gas.

The first group will be examined below. In a chemical reaction in which NH₃ gas combusts with Cl₂ gas as a combustion supporting gas, NH₃ gas and Cl₂ gas are materials at the raw material side and NH₄Cl (ammonium chloride) and N₂ gas materials at the product side as illustrated in the following reaction formula (1). 8NH₃+3Cl₂→6NH₄Cl+N₂  (1)

In this chemical reaction, an N—H bond before reaction is broken and an N—N bond is generated after reaction. N—H bond dissociation energy is 386 kJ/mol and N—N bond dissociation energy is 941 kJ/mol. Thus, comparing bond dissociation energy of a raw material side with bond dissociation energy of a product side, bond dissociation energy of the product side is significantly greater than bond dissociation energy of the raw material side and a difference therebetween is 555 kJ/mol.

Next, a chemical reaction in which C₂H₄ gas combusts with Cl₂ gas as a combustion supporting gas may be referred to as an addition reaction in which a halogen gas is added to alkene as illustrated in the following reaction formula (2). That is, C₂H₄ gas and Cl₂ gas are materials at the raw material side and CCl₄ (carbon tetrachloride) and HCl are materials at the product side. C₂H₄+6Cl₂→2CCl₄+4HCl  (2)

Taking note of variation before and after reaction, it is found that a C═C double bond and a C—H bond at a raw material side are replaced with a C—Cl bond and an H—Cl bond at a product side.

Bond dissociation energy of a C═C double bond is 418 kJ/mol and C—H bond dissociation energy is 419 kJ/mol. In addition, C—Cl bond dissociation energy is 342 kJ/mol and H—Cl bond dissociation energy is 428 kJ/mol. Accordingly, comparing the higher bond dissociation energy at the raw material side, that is, 419 kJ/mol, with the higher bond dissociation energy at the product side, that is, 428 kJ/mol, bond dissociation energy of the product side is greater than bond dissociation energy of the raw material side by 9 kJ/mol.

As described above, in a chemical reaction formula of a reaction between a combustible gas (NH₃ and C₂H₄) belonging to the first group and Cl₂ gas, bond dissociation energy of a product side becomes obviously greater than bond dissociation energy of a raw material side (by at least several % or more).

Next, the second group will be examined. A flame ignition phenomenon in a case where a combustible gas (CH₄, CH₃F and CH₂F₂) belonging to the second group is mixed with Cl₂ gas is considered radical substitution. That is, it can be said that gas molecules of Cl₂ gas do not directly participate in a chemical reaction with a combustible gas but a chlorine radical Cl* directly participates in the chemical reaction. In addition, in the present specification, a radical is denoted by appending a superscript * to an atomic symbol.

A chemical reaction in which methane (CH₄) gas combusts with a chlorine radical Cl* as a combustion supporting gas is a reaction in which a C—H bond is broken and both C and H are subjected to halogenation as illustrated in the following reaction formula (3). CH₄+8Cl*→CCl₄+4HCl  (3)

In this chemical reaction, both CCl₄ (carbon tetrachloride) and HCl (hydrogen chloride) as products are gases. In this reaction, all C—H bonds are broken and a C—Cl bond and an H—Cl bond are generated. At a raw material side, C—H bond dissociation energy is 432 kJ/mol. On the other hand, at a product side, H—Cl bond dissociation energy is 428 kJ/mol and C—Cl bond dissociation energy is 342 kJ/mol. Comparing the higher bond dissociation energy (432 kJ/mol) at the raw material side with the higher bond dissociation energy (428 kJ/mol) at the product side, a difference therebetween is at most 4 kJ/mol (less than 1%). That is, it can be said that bond dissociation energy at the raw material side is approximately equal to bond dissociation energy at the product side.

A chemical reaction in which CH₃F gas combusts with a chlorine radical Cl* as a combustion supporting gas is a reaction in which a C—H bond is broken and both C and H are subjected to halogenation, in the same manner as methane gas, as illustrated in the following reaction formula (4). CH₃F+6Cl*→CCl₃F+3HCl  (4)

In this chemical reaction, both CCl₃F and 3HCl as products are gases. In this chemical reaction, all C—H bonds are broken and a C—Cl bond and an H—Cl bond are generated. In addition, in the same manner as CH₄ gas, a raw material side has a C—H bond having bond dissociation energy of 432 kJ/mol and a product side has an H—Cl bond having bond dissociation energy of 428 kJ/mol and, therefore, a difference therebetween is at most 4 kJ/mol (less than 1%). That is, it can be said that bond dissociation energy at the raw material side is approximately equal to bond dissociation energy at the product side.

In addition, in this reaction, a C—F bond cannot be broken by a chlorine radical Cl*. This is because fluorine has stronger reactivity than chlorine.

A chemical reaction in which CH₂F₂ gas combusts with a chlorine radical Cl* as a combustion supporting gas is a reaction in which a C—H bond is broken and both C and H are subjected to halogenation, in the same manner as CH₄ gas and CH₃F gas, as illustrated in the following reaction formula (5). CH₂F₂+4Cl*→CCl₂F₂+2HCl  (5)

In this chemical reaction, both CCl₂F₂ and 2HC1 as products are gases. In this reaction, all C—H bonds are broken and a C—Cl bond and an H—Cl bond are generated. In addition, in the same manner as CH₄ gas, bond dissociation energy of a C—H bond is 432 kJ/mol at the raw material side, whereas bond dissociation energy of an H—Cl bond is 428 kJ/mol at the product side. A difference therebetween is very small (less than 1%) and it can be said that bond dissociation energy of the raw material side is approximately equal to bond dissociation energy of the product side.

In addition, in this reaction, a C—F bond cannot be broken by a chlorine radical Cl*, in the same manner as CH₃F gas. This is because fluorine has stronger reactivity than chlorine.

As described above, in a chemical reaction formula of a reaction between a combustible gas (CH₄, CH₃F and CH₂F₂) belonging to the second group and Cl₂ gas, bond dissociation energy of the raw material side becomes approximately equal to bond dissociation energy of the product side.

Next, the third group will be examined. As described above, a combustible gas (COS, CO and C₄F₆) belonging to the third group does not react when mixed with Cl₂ gas as a combustion supporting gas and is not ignited even by provision of flame. However, under some conditions, for example, under plasma, interatomic bonding is relatively easily broken and resulting atoms are provided to a process.

In a chemical reaction of COS gas and Cl₂ gas, COS gas at the raw material side has a C═O double bond and a C═S double bond and bond dissociation energies of these double bonds are 1072 kJ/mol and 572 kJ/mol, respectively, which are obviously greater (by at least several % or more) than C—Cl bond dissociation energy (342 kJ/mol) in a compound that may be obtained at the product side, for example, CCl₄.

In a chemical reaction of CO gas and Cl₂ gas, CO gas at the raw material side has a C═O double bond and bond dissociation energy of the double bond is 1072 kJ/mol. Thus, the bond dissociation energy of the raw material side is obviously greater (by at least several % or more) than C—Cl bond dissociation energy of 342 kJ/mol in a compound that may be obtained at the product side, for example, COCl₂.

In a chemical reaction of C₄F₆ gas and Cl₂ gas, C₄F₆ gas at the raw material side has a C═C double bond and a C—F bond and bond dissociation energies of these bonds are 719 kJ/mol and 472 kJ/mol, respectively. Thus, bond dissociation energies at the raw material side are obviously greater (by at least several % or more) than C—Cl bond dissociation energy (342 kJ/mol) at the product side.

As described above, in a chemical reaction formula of a reaction between a combustible gas (COS, CO and C₄F₆) belonging to the third group and Cl₂ gas, bond dissociation energy at the product side becomes obviously smaller than bond dissociation energy at the raw material side (by at least several % or more).

[Molecular Structure of Combustible Gas Belonging to Second Group]

FIG. 4 illustrates the molecular structure, chemical reaction formula, lowest explosion concentration, bond dissociation energy at the raw material side, and bond dissociation energy at the product side for each of combustible gases (CH₄, CH₃F, and CH₂F₂) belonging to the second group in the evaluation test.

As illustrated, the molecular structure of CH₄ gas is a structure in which hydrogen (H) atoms are joined respectively to four bonding hands of a carbon (C) atom. The molecular structure of CH₃F gas is a structure in which one of four hydrogen atoms in the molecular structure of CH₄ gas is replaced with a fluorine (F) atom. The molecular structure of CH₂F₂ gas is a structure in which two of four hydrogen atoms in the molecular structure of CH₄ gas are replaced with fluorine (F) atoms.

In the row “reaction formula” of FIG. 4, the same chemical reaction formulas as the above-described reaction formulas (3), (4) and (5) are indicated. Here, although Cl at the raw material side is provided in the form of gas molecules (Cl₂), Cl in the form of a chlorine radical Cl* participates in a reaction because of actually small Cl—Cl covalent character. Although both fluorine (F) and chlorine (Cl) are halogen elements having high reactivity, fluorine (F) has higher reactivity than chlorine (Cl). Thus, a chlorine radical Cl* breaks a C—H interatomic bond and acts to bond with a C atom. As a result, a reaction as illustrated by chemical reaction formula (3) occurs. However, chlorine (Cl) cannot break a C—F bond because chlorine (Cl) has lower reactivity than fluorine (F). That is, a C—F bond is not replaced with a C—Cl bond because bond dissociation energy (489 kJ/mol) of a C—F bond is higher than bond dissociation energy (342 kJ/mol) of a C—Cl bond.

In FIG. 4, the row “raw material side (bond dissociation energy)” illustrates one or plural interatomic bonds, which are present before a chemical reaction and are not present after the chemical reaction, and bond dissociation energies of the respective bonds. In addition, the row “product side (bond dissociation energy)” illustrates plural interatomic bonds, which are not present before a chemical reaction and are present after the chemical reaction, and bond dissociation energies of the respective bonds. Interatomic bonds having no variation before and after a reaction are excluded. An interatomic bond refers to an interatomic bond having a great covalent character. Thus, ions or radicals are excluded.

As illustrated, as a common characteristic or property of combustible gases (CH₄, CH₃F, and CH₂F₂) belonging to the second group, there are ones having approximately the same bond dissociation energy (having a difference of less than 1%) between all interatomic bonds of a raw material side and all interatomic bonds of a product side. That is, the raw material side has a C—H bond (432 kJ/mol) and the product side has an H—Cl bond (428 kJ/mol).

In FIG. 5, for combustible gases (CH₄, CH₃F, and CH₂F₂) belonging to the second group, a characteristic curve (regression curve) of the lowest explosion concentration acquired by regression analysis based on plots obtained from the evaluation test is illustrated in a graph whose the horizontal axis represents the number of C—H bonds and the vertical axis represents the concentration of a combustible gas. As described above, the number of C—H bonds in CH₄, CH₃F, and CH₂F₂ are 4, 3 and 2, respectively, and the lower explosion limits of CH₄, CH₃F, and CH₂F₂ (lowest explosion concentrations) are greater than 0.10%, greater than 5% and greater than 13.3%, respectively. A tertiary function represented by the following Equation (6) may be acquired by acquiring a regression curve, which passes the above three plots and a fourth plot representing that the lower explosion limit logically becomes 100% when the number of C—H bonds is 0 (zero), via regression analysis. y=−2.4958x ³+24.162x ²−81.692x+100  (6)

This graph means that, with regard to the number of C—H bonds determined from the molecular structure of a combustible gas belonging to the second group, the combustible gas is not ignited when the concentration of the gas is in a region below the characteristic curve and is ignited when the concentration of the gas is in a region above the characteristic curve.

[Management of Prevention of Explosion of Exhaust Gas in Exemplary Embodiment]

In the decompression processing apparatus (more particularly, the plasma processing apparatus) to which the present disclosure may be applied, based on knowledge obtained from the results of the evaluation test (in particular, classification into three groups) and numerical value information (in particular, the lower explosion limit of combustible gases belonging to the second group), management of prevention of explosion of an exhaust gas may be appropriately and efficiently performed.

That is, with regard to combustible gases (NH₃ and C₂H₄) belonging to the first group, the combustible gases combust by being merely mixed with Cl₂ gas and, therefore, it is necessary to prevent mixing of both the gases in a gas supply system (processing gas supply system) as well as an exhaust system. Thus, in an actual process, when there is a probability that combustible gases (NH₃ and C₂H₄) belonging to the first group and Cl₂ gas are mixed in the processing gas supply unit 24, the main control unit 42 may control an interlock such as, for example, operation stop or shutdown, based on recipe information.

With regard to combustible gases (CH₄, CH₃F, and CH₂F₂) belonging to the second group, the combustible gases do not cause explosion or reaction when merely mixed with Cl₂ gas. Therefore, in an actual process, a recipe that supplies a mixed gas of both the gases, as a processing gas, from the processing gas supply unit 24 to the chamber 10 may be allowed. However, the dilution unit 57 of the exhaust gas processing unit 40 may be operated because there is a risk of gas leaking from the exhaust pipe when an exhaust gas including unreacted combustible gases (CH₄, CH₃F, and CH₂F₂) and Cl₂ gas explodes at the outlet side of the vacuum pump 36 due to, for example, static electricity.

In this case, the lower explosion limit acquired from the evaluation test may be set to a reference value or target value for exhaust gas dilution control according to the kind of a combustible gas. That is, when an exhaust gas discharged from the vacuum pump 36 includes CH₄ and Cl₂, the supply flow rate of a dilution gas, i.e. an inert gas supplied from the dilution unit 57 to the exhaust pipe 46 is controlled such that the concentration of CH₄ may be kept at or near a value lower than the lower explosion limit and may be in a range of 0.05% to 0.10% and more particularly in a range of 0.07% to 0.10%.

In addition, when an exhaust gas discharged from the vacuum pump 36 includes CH₃F and Cl₂, the supply flow rate of an inert gas supplied from the dilution unit 57 to the exhaust pipe 46 is controlled such that the concentration of CH₃F may be kept at or near a value lower than the lower explosion limit and may be in a range of 3% to 5% and more particularly in a range of 4% to 5%.

In addition, when an exhaust gas discharged from the vacuum pump 36 includes CH₂F₂ and Cl₂, the supply flow rate of an inert gas supplied from the dilution unit 57 to the exhaust pipe 46 is controlled such that the concentration of CH₂F₂ may be kept at or near a value lower than the lower explosion limit and may be in a range of 8.3% to 13.3% and more particularly in a range of 10.3% to 13.3%.

By performing the exhaust gas dilution control as described above, consumption of an inert dilution gas used to prevent explosion of an exhaust gas, in advance, in the exhaust pipe 46 may be restricted to a required minimal level.

With regard to combustible gases (COS, CO and C₄F₆) belonging to the third group, the combustible gases have no risk of a combustion reaction or ignition even if an exhaust gas discharged from the vacuum pump 36 includes these combustible gases and Cl₂ gas. Therefore, it is unnecessary to operate the dilution unit 57 of the exhaust gas processing unit 40.

[Other Exemplary Embodiments or Alternative Embodiments]

Even with regard to combustible gases other than the seven kinds of combustible gases (CH₄, CO, CH₃F, CH₂F₂, C₄F₆, COS and C₂H₄) selected in the evaluation test and NH₃ gas, in terms of the above-described bond dissociation energy, i.e. based on comparison (magnitude relationship) of bond dissociation energy between a raw material side and a product side in a chemical reaction formula of the combustible gases and Cl₂ gas, it may be determined which one of the first group, the second group and the third group the combustible gas belongs to and, based on the determined result, the same solution for preventing explosion of an exhaust gas as the above description may be conceived.

In addition, based on analysis results in terms of bond dissociation energy as described above, the same method as the above description may be applied even when, for example, F₂ (fluorine) gas or Br₂ (brome) gas that is also a halogen gas is used instead of Cl₂ gas as a combustion supporting gas.

In the evaluation test, a case where a combustible gas and a combustion supporting gas are directly mixed with each other, i.e. a case where a mixed gas of both gases is supplied to a chamber was evaluated. However, an actual decompression processing apparatus may alternately supply a combustible gas and a combustion supporting gas into a chamber in a successive process. For example, a plasma CVD apparatus may alternately supply a combustible gas for film formation and a combustion supporting gas for plasma cleaning to a chamber. Even in this case, when some of the combustible gas used for a film forming processing remains unreacted in the chamber and is mixed with the combustion supporting gas during subsequent plasma cleaning to thereby be discharged from the chamber, a high-pressure or high-concentration exhaust gas may cause explosion (rapid variation in pressure by combustion) at the outlet side of the vacuum pump due to, for example, generation of static electricity. Accordingly, in a case where the same relationship as in the second group is established between the combustible gas and the combustion supporting gas which are alternately supplied to the chamber, the dilution unit 57 of the exhaust gas processing unit 40 may be operated upon execution of plasma cleaning.

In addition, in the exhaust gas processing unit 40, an end of the dilution gas supply pipe 50 may be connected to an exhaust path in the vacuum pump 36 instead of the exhaust pipe 46. In addition, a gas sensor (not illustrated) may be attached to the exhaust pipe 32 at the inlet side of the vacuum pump 36 to measure the concentration of a combustible gas before the combustible gas reaches a high pressure state at the outlet side of the vacuum pump 36 and the dilution controller 45 or the dilution unit 57 may be operated based on the measured upstream concentration value.

As described above, the present disclosure enables management of a process under a condition in which a combustion reaction does not occur by controlling the concentration of a gas even if the gas is a mixed gas of a combustible gas and a combustion supporting gas and may be appropriately applied to a process (in particular, an etching process) in which a combustible gas and a combustion supporting gas are simultaneously supplied to a processing container.

From the foregoing, it will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be made without departing from the scope and spirit of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting, with the true scope and spirit being indicated by the following claims. 

What is claimed is:
 1. A method for preventing explosion of an exhaust gas in an exhaust path at an outlet side of a vacuum pump included in a decompression processing apparatus, the method comprising: supplying a mixed gas of a combustible gas containing either substituted or unsubstituted methane and a combustion supporting gas containing a halogen element to a chamber of the decompression processing apparatus thereby performing a decompression processing on a processing target substrate at a reduced pressure while evacuating the chamber with the vacuum pump; measuring a concentration of the combustible gas in the exhaust path to determine a flow rate of the inert as supplied to the exhaust path or to the vacuum pump; and supplying an inert gas to the exhaust path or to the vacuum pump at the determined flow rate such that a concentration of the combustible gas in the exhaust gas is maintained at a value lower than a predetermined limit.
 2. The method according to claim 1, wherein the flow rate of the inert gas supplied to the exhaust path is determined based on a kind and combination of the mixed gas.
 3. The method according to claim 1, wherein the concentration of the combustible gas is determined by a number of C—H.
 4. The method according to claim 1, wherein the combustible gas includes CH₄ (methane).
 5. The method according to claim 4, wherein the CH₄ gas in the exhaust path has a concentration in a range of 0.05% to 0.10%.
 6. The method according to claim 4, wherein the flow rate of the inert gas supplied to the exhaust path is controlled such that the concentration of the CH₄ gas is in a range of 0.07% to 0.10%.
 7. The method according to claim 1, wherein the combustion supporting gas is C1₂ (chlorine) gas. 